power: reset: syscon-poweroff: add a mask property

Make the syscon-poweroff driver accept value and mask instead of
just value.

Prior to this patch, the property name for the value was 'mask'. If
only the mask property is defined on a node, maintain compatibility
by using it as the value.

Signed-off-by: Guy Shapiro <guy.shapiro@mobi-wize.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sebastian Reichel <sre@kernel.org>